Shorten repeated module-prefix filenames

[yvette-carlisle]

Summary

• shorten repeated module-prefix Rust filenames by moving context into module directories
• keep long artifact/spec/fixture names where the filename is the external contract
• update module declarations and test paths after renames

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo nextest run -p elf-eval --all-targets --all-features
• git diff --cached --check